Directing queries through forwarders
Updated: January 21, 2005
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
Directing queries through forwarders
A DNS server that acts as a forwarder may serve other purposes in the DNS environment. For example, it could be a primary or secondary server, or it could be a caching-only server. Consider the activities of all DNS servers in your network when deploying forwarders to ensure that forwarding complements the other DNS server activities and benefits name resolution. For general information about forwarders, see Understanding forwarders.
Forwarders and delegation
A DNS server configured with a forwarder and hosting a parent zone will use its delegation information before forwarding queries. If no delegation record exists for the DNS name in the query, then the DNS server will use its forwarders to resolve the query.
Forwarders and root servers
A common error when configuring forwarding is to attempt to configure forwarding on root servers for a private DNS namespace. Root servers are authoritative for the root zone of your organization's DNS namespace. The goal of attempting to configure forwarding on root servers for a private DNS namespace is to forward all offsite queries to Internet DNS servers. Root servers cannot be configured with standard forwarding. If a root server is queried about any domain name, then it will refer to a DNS server that can answer the question (from its local zones, cache or root hints), or it will respond with a failure (NXDOMAIN), but it cannot be configured to forward to specific servers.
A root server can be configured with a conditional forwarder. Conditional forwarding can be used to forward queries between root servers in separate DNS namespaces. For steps to configure internal root servers, see article 294906 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?LinkID=82900). For more information about configuring conditional forwarding, see article 304491 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?LinkID=82901).
Forwarder configuration tips
Consider the following tips for efficient forwarder configuration and use:
Keep forwarder configuration uncomplicated. For every DNS server configured with a forwarder, queries can be sent to a number of different places. Each forwarder and each conditional forwarder must be administered for the benefit of DNS client queries, and this process can be time consuming. Use forwarders strategically, where they are needed the most, such as resolving offsite queries or sharing information between namespaces.
Avoid chaining your forwarders. If you have configured a DNS server named server-1 to forward queries for widgets.example.com to DNS server server-2, do not configure server-2 to forward queries for widgets.example.com to DNS server server-3. This is an inefficient resolution process and could result in errors if server-3 is accidentally configured to forward queries for widgets.microsoft.com to server-1, in which case you would have a chain.
Do not concentrate too great of a load on forwarders. The recursive queries that forwarders send to the Internet can require a significant amount of time to answer due to the nature of the Internet. With a large number of internal DNS servers using these forwarders for Internet queries, they can experience a substantial concentration of network traffic. If network load will be an issue, use more than one forwarder and distribute the load between them.
Do not create inefficient resolution using forwarders. The DNS server will attempt to forward domain names according to the order in which they are configured in the DNS console. A DNS server in Seattle may be incorrectly configured to forward a query to a server in London instead of another server in Seattle because the server in London is higher in the forwarders list. This will decrease the efficiency of name resolution on the network. Evaluate your network's forwarding configuration periodically to see if there are similar configurations.
For more information, see Configure a DNS server to use forwarders.